Optical communication systems employ wavelength combining techniques known as wavelength division multiplexing (WDM) to multiplex many transmission channels onto a single-mode fiber. Non-linear transmission effects in optical fibers, however, may cause interactions between adjacent optical channels, which limit performance of such systems. One way to reduce these cross-channel interactions is by multiplexing the channel wavelengths such that the adjacent frequencies in the channel spectrum propagate along the transmission fiber with orthogonal states of polarization. To launch orthogonal WDM signals, existing systems often use polarization maintaining (PM) components or use complex polarization control techniques when PM components are not commercially available or viable. As PM components become more available, such components enable practical implementations of orthogonal launch of WDM signals without complex polarization control.
Other techniques may also be used to avoid crosstalk penalty in dense wavelength division multiplexed (DWDM) systems having high spectral efficiencies. One such technique involves pre-filtering the individual data spectra prior to multiplexing the channel wavelengths onto a transmission fiber. In orthogonally launched DWDM systems, however, providing both pre-filtering and orthogonal launch presents unique challenges particularly when PM components are used. Using separate PM comb filters to pre-filter the separate groups of wavelengths to be orthogonally combined, for example, may degrade system performance because of difficulties aligning the frequency and pass-band shape of multiple PM comb filters on the transmit side and an interleaver on the receive side to the data channels. These problems associated with controlling the filter shapes and center frequency alignment are compounded in systems with several groups or bands of wavelengths being multiplexed together and thus having many filters. Moreover, using additional PM components may degrade system performance because of the polarization crosstalk effect that results within PM devices and connectors from the stress in the ferrules and arrays that hold PM fibers inside of PM devices.